As a result of pollution and environmental problems, retail gasolines in the United States will eventually have a phased down lead content. Maintenance of high octane gasolines by methods other than lead addition is of continuing interest to U.S. refiners. Three major techniques are available to acquire high octane gasoline pools without lead addition. First, hydrocarbons can be reformed in the presence of a reforming catalyst, such as a platinum-rhenium catalyst, to a high octane gasoline. Second, alcohol-based oxygenates, such as methyl tertiary butyl ether (MTBE) can be added to motor gasoline to obtain enhanced octane ratings. Third, normal paraffins can be isomerized to branched paraffins possessing high octane qualities. The present invention concerns the third process and is an improvement over prior art isomerization process.
In Holcombe, U.S. Pat. No. 4,210,771, a basic process referred to as a Total Isomerization Process (TIP) is disclosed in which a feedstream consisting essentially of mixed normal and branched hydrocarbons is passed first through an isomerization reactor and the products therefrom passed to an adsorption section which separates normal from branched paraffins to form an isomerate having di- and mono-branched paraffins and a recycle stream of nearly pure normal paraffins. The resultant gasoline therefore contains mono-branched hexanes which have an inherently lower octane rating than the di-branched paraffins and the mono-branched pentane.
A second Holcombe patent, U.S. Pat. No. 4,176,053, discusses a normal paraffin-isomerization separation process. A molecular sieve is employed to separate normal paraffins from a feedstock mixture comprising normal and branched paraffins, and only normal paraffins are adsorbed and fed to the isomerization reactor after void space purging of the molecular sieve absorbent.
U.S. Pat. No. 4,717,784, issued to Evans et al on Jan. 5, 1988, discloses a process for the isomerization of feed streams comprising C.sub.6 or C.sub.6.sup.+ normal paraffins with recycle of mono-methyl-branched paraffins and normal paraffins. A separation zone downstream of the isomerization reactor is comprised of a sieve having a pore size between 5.5.times.5.5 .ANG. and 4.5.times.4.5 .ANG. which permits the adsorption of mono-methyl-branched paraffins and normal paraffins while allowing the dimethyl paraffins to pass through the sieve and be collected as the isomerate product stream. Both normal olefins and all the mono-branched olefins are absorbed in the separation zone and recycled to the isomerization zone. If the feed stream contains pentanes, this process would result in the recycling of high-octane-rated mono-branched pentane and would therefore result in a build up of mono-branched pentane in the process. Thus, the feedstocks claimed in the process taught by Evans are limited to C.sub.6 or greater carbon atoms. Since most commercial feedstocks contain pentanes, the process is therefore not commercially viable.
A second Evans patent, U.S. Pat. No. 4,804,802, issued on Feb. 14, 1989, is an improvement on the separation technique used in the isomerization process taught in the above-mentioned Evans, U.S. Pat. No. 4,717,784. The separation zone downstream of the isomerization reactor is comprised of a first separatory sieve having a pore size smaller than or equal to 4.5.times.4.5 .ANG. to adsorb normal paraffins while allowing mono-methyl-branched paraffins and di-branched paraffins to pass through the sieve. The second sieve is a molecular sieve having a pore size greater than 5.5.times.5.5 .ANG. and less than 4.5.times.4.5 .ANG. which will adsorb the mono-methyl-branched paraffins while allowing the dimethyl paraffins to pass through the sieve and be collected as the isomerate product stream. The feedstocks claimed in the process are again limited to C.sub.6 or greater carbon atoms. As discussed above, if the feed stream contains pentanes, this process would also result in the recycling of high-octane-rated mono-branched pentane and would therefore result in a build up of mono-branched pentane in the process which would likewise not be commercially viable.
A third Evans patent, U.S. Pat. No. 4,855,529, issued on Aug. 8, 1989, is an improvement process upon the isomerization process taught in the above-mentioned Evans, U.S. Pat. No. 4,804,802. The process is similar to that taught in the U.S. Pat. No. 4,855,529 except that the di-branched paraffins in the feedstocks are separated prior to the isomerization step. The process is likewise not commercially feasible for the isomerization of pentane containing feedstocks.
The feedstream to a Total Isomerization Process is usually derived from refinery operations and normally is comprised mainly of isomeric forms of saturated hydrocarbons having five and six carbon atoms. After isomerization, the product stream is comprised primarily of n-pentane, isopentane (mono-branched C.sub.5), n-hexane, dimethylbutanes (di-branched hexanes), and methylpentanes (mono-branched hexanes). As illustrated in the TABLE 1 below, di-branched hexanes and mono-branched pentane (isopentane) have high octane numbers. However, mono-branched hexanes (methylpentanes) have relatively low octane numbers. It is, therefore, advantageous to develop a process which would recycle the low-octane-rated methylpentanes and normal paraffins, but recover, without recycling, the high-octane-rated isopentane and di-branched hexanes.
TABLE 1 ______________________________________ The ASTM C.sub.5 and C.sub.6 Octane Numbers ASTM Research Octane Numbers Component (0.0 mL TEL/gal) ______________________________________ n-Pentane 62 2-Methylbutane (isopentane) 93 n-Hexane 25 2-Methylpentane 73 3-Methylpentane 75 2,2-Dimethylbutane 92 2,3-Dimethylbutane 104 ______________________________________
In the Total Isomerization Process (TIP) taught by Holcombe, di-branched hexanes are recovered along with all the mono-branched paraffins and only normal paraffins are recycled to the isomerization zone. The resultant gasoline contains mono-branched hexane which has an inherently low octane rating as indicated in the TABLE 1 above.
In the prior art processes disclosed by Stem et al, both normal paraffins and all the mono-branched paraffins are absorbed in a molecular sieve type separation zone and recycled to the isomerization zone. If the feed stream contains pentanes, this process would result in the recycling of high-octane-rated mono-branched pentane and would therefore result in a build up of mono-branched pentane in the process. Since most commercial feedstocks contain pentanes, this process is therefore not commercially viable.
Accordingly, there remains a demand for an integrated isomerization process that is commercially viable for converting a C.sub.5 and C.sub.6 containing feedstocks to an isomerate gasoline blending component with higher octane number and which process would recover both the di-methyl-branched paraffins and mono-methyl-branched pentane (i.e. isopentane) in the isomerate, but recycle the low octane rated mono-methyl-branched hexanes (i.e. methylpentanes) and normal paraffins. The instant invention provides such a process.